thanks @PaulOB
I managed to find a firefox specific rule which corrected everything.
@-moz-document url-prefix() {
a.soft {
background-color: #ddd;
box-shadow: -10px -10px 20px 0 #E6E6E6,
10px 10px 20px 0 #ABABAB,
inset 10px 10px 20px 0 #E6E6E6,
inset -10px -10px 20px 0 #ABABAB;
color: #888;
}
}